Components within the Cell Membrane | Microbiology

Cytoplasm:

Cell membrane encloses a space with hyloplasm or cytosol, which is the ground substance and sheat of all metabolic activities. Cytoplasm consists of water, protein, including multifunctional enzymes, lipid, carbohydrates, different types of RNA molecules and various smaller molecules.

Cytoplasm of bacteria is differentiated into two distinct areas. A less electron dense nuclear material and a dark region. In dense cytoplasm occurs thousands of particles about 25nm in diameter called ribosomes which are composed of ribonucleic acid and proteins.

They are site of protein synthesis. Ribosomes of bacteria are 70S type and consist of two subunits (i.e., longer 50S ribosomal subunit and smaller 30S ribosomal subunit). Nonfunctional ribosomes exist in the form of separated subunit which are suspended freely in cytoplasm. During protein synthesis many ribosomes lead the code of single m RNA and form polysome or poly ribosomes.

Reserve material of bacteria are also stored in cytoplasm either as finally dispersed or distinct granules called inclusion bodies or storage granules. These are organic polymers which either serve as reserve of PHB (Poly β hydroxy butyric acids) or as store of energy or as the polymers of glucose called granulose (i.e., glycogen).

Nucleoid:

In bacteria, the nuclear material includes, a single circular and double stranded DNA molecule which is called bacterial chromosome. It is not separated from cytoplasm by nuclear membrane as it occurs in eukaryotic cell. However, nuclear material concentrated in a clear region of cytoplasm is called nucleoid. A nucleoside has no ribosomes and nucleolus.

Cell Membrane Intrusions:

Bacterial cells do not contain membrane enclosed organelles corresponding to the mitochondria and chloroplast of eukaryotic cell. However, bacteria may have specialized invagination of cell membrane that increase their surface area for certain functions.

Mesosomes:

Many bacteria, especially, Gram positive bacteria, possess membrane invagination in the form of convoluted tubules and vesicles termed as mesosomes. They are seen in chmoautophic bacteria with high rate of aerobic respiration such as nitrosomonas. Mesomomes are involved in cross link due to the division of cell.

Cytoskelton:

Eukaryotic cell contains different classes of fibrous structures collectively termed cytoskelton elements which are made up of tubulin containing microtubule, microfilament and intermediate fibre made of an actin protein.

There are no structures in bacteria homologous with any of the three classes of cytoskeltal elements of eukaryotes. A short tubular structure with approximate dimension of microterbules has been seen in bacteria but these are most probably fragments of bacterial viruse, many of which have tubular structure. Beside lacking demonstrable cytoskeltal elements, bacteria lack sufficient protein similar to actin and tuberlin.

It has been suggested that endoflagellum of spirochetes is homologous with eukaryotic microtubules. Indeed, ail bacterial flagella closely resemble microtubules (they are hollow tubes about 15 nm in diameter).

Comparison of Typical Eubacterial Cell Structure with Archeal and Eukaryotes:

The numerous and profound differences of organization and function among three cell types -eukaryotes, eubacteria and archaebacteria have been revealed fully recognized.

They differ from each other with respect to the structure, and mechanism mediating a variety of fundamental cellular activities such as transmission, transcription and translation of genetic material, the structure of their chromosomes, organization of electron transport system, organization of cytoplasm, structure of cytoplasmic membrane, nutrient uptake, secretion and movement.

These differences are summarized in Table-10.1.

The Archaea:

Archaeabacteria are heterogenous group that are phylogenetically very distant from eubacteria.

Their distinguishing properties such as ether link containing isporenoid side chain in contrast to ester link hydrocarbon in all other biological systems, subtantial differences between them and eubacteria with respect to subunit structure of RNA polymerase, lack of muramic acid as constituent of cell wall peptidoglycan in eubacteria, lack of ribothymin in t RNA.

The archaea include microbes found in two phyla; the crenaschaeoto and Euryarchaeota. Archaea are quite diverse, both in morphology and physiology. They can stain either Gram positive or Gram negative and may be spherical, rod shaped, spiral, lobed, irregular and plemorphic.

They range in diameter from 0.11 to 16 µm and some filamentous can grow up-to 200 µm in length. Multiplication may be by binary fission, budding and fragmentation. The archaea are just as diverse physilogically. They can be aerobic, facultative anaerobes or strictly anaerobes.

Nutritionally they range from chemolitho autotrophs to organotrophs. They include Psychrophiles, mesophiles and hyper theromophiles that can grow above 100^oC.

Archaea have most often been found in area with either very high or low temperature or pH concentrated salts or completely anoxic. They are referred to as extreme environments.

Archaeal Taxonomy:

Now, it is clear that archaeal taxonomy are quite distinct from other living organisms. Archaea can be divided in to five major groups based on physiological and morphological differences.

On the basis of phylogenetic evidence, Bergey’s Manual divides Archaea in­to the phyla Euyarchaeota and crenarchaeota. The eury-archaeotes are given this name because they occupy many different ecological niches and have a variety of metabolitic pattern.

The phylum euryacrhaeota is very diverse with eight classes (Methanobacteria, Methanococci, Halobacteria, Thumoplasmata, Thermococci, Archaeglobi, Methanopyri and methano microbia) nine orders and 16 families.

The methanogens, extreme haiophiles, sulfate reducers and many extreme thermophiles with sulphur-dependent metabolism are located in euryarchaeota. Methanogens are dominant physiological group.

The crenarchaeotes resemble the ancestor of archaea, and it includes well characterized species thermophiles or hyperthermophiles. Crenarchaeota has only one class thermoprotei, which is divided into four orders and six families. Order thermoproteales contains garm negative, anaerobic to facultative, hyperthermophilic rods.

They grow chemolithotrophically by reducing sulphur to hydrogen sulphides. Members of the order sufolobales are coccus shaped thermoacidophilels. The order desulfurococcales contain gram-ve coccoid and disk shaped hyperthermophites hyperthermophytes.

Rickettsias:

The rickettsias are small, gram-negative, coccoid or rod shaped (usually appear as rods with rounded edges, a form known as the coccobacillus) or pleomorphic bacteria with typical gram negative walls and no flagella, pilli, capsule or spore. Al-thought their size varies, these bacterial tend to be very small. For example, Rickettsia is 0.3 to 0.5 µm in diameter and 0.8 to 2.0 µm long. Coxiella is 0.2 to 0.4 µm by 0.4 to 1.0 µm.

They are obligate intracellular parasites and have not been cultivated in the absence of host cell. Rickettsias are causative agent of such human diseases as typhus fever, Rocky mountain spotted fever and Q fever.

Rickettsias show cell with a normal bacterial morphology; Both cell wall and cell membrane are visible. The cell wall contains muramic acid and diaminopalmilic acid. Both RNA and DNA are present and rickettsias are divided by normal binary fission, with doubling time of 8 hours.

Penetration of host cell by rickettsias cell is an active process, requiring both host and parasite to a live and metabolically active. Once inside the host phagocytic cell, the bacteria multiply primarily in the cytoplasm and continue replicating until the host cell is loaded with parasite, at this time the host cell bursts and liberates the bacteria into the surrounding fluid.

Metabolism and Pathogenesis:

Many rickettsias possess a highly distinctive energy metabolism, being able to oxidize only glutamate or glautamins and being unable to oxidize glucose or organic acids. However, coxiell burnett is able to utilize both glucose and pyruvate as electron donor.

Rickettsias posses a respiratory chain complete with cytochrome and are able to carry out electron transport phsophorylation using NADH as electron donor. They are also able to synthesize at least some of the small molecules needed for macromolecule synthesis and growth, and they obtain the rest of their nutrients from host cell.

Rickettsias depend upon host cell for some essential metabolic reaction but they are capable of carrying limited metabolic reach independently. Rickettsias all free from host cell rapidly lose their viability, Loss of viability may be retained by addition of Co enzyme NADS CoA which are to be supplied by host cell during multiplication.

(1) Typhus fever:

Fever, headache and body weakness- transmitted from human to human by common body or headache.

(2) Spotted lever:

Rocky mountain spotted fever was first recognized in the Western United States about 1900 but more common today in South Eastern United State.

(3) Ehrlichiosis:

Fever, headache and Leukoponia (decreased number of leucocytes).

(4) Q. Fever:

Pneumonia like injection

Not transmitted to human directly by insect bite.

Rickettsias do not survive long outside their hosts and this may explain why they must be transmitted from animal to animal by arthropod vectors.

When the arthropod obtains a blood meal from an infected vertebrate, Rickettsias present in the blood are inoculated directly into arthropods, where they penetrate into epithelial cells of gastrointestinal tract, multiply and appear later in the faeces. When the arthropod feeds on an uninfected individual, it then transmits the rickettsias either directly with its mouth parts or by contaminating the bite with its faces.

Causal agent of Q fever, Coxiella burnetti, can also be transmitted to the respiratory system by aerosols. Coxielia burnetti is the most resistant of the rickettsias to physical damage, probably because it produces a resistant, spore like form, and this explains its ability to survive in air.

Rochalimaea is a typical rickettsias because it can be grown in culture and is thus not an obligate intracellular parasite. In addition, when growing in tissue culture, cells of Rochalimaea grown on the outside surface of the eukaryotic host cells rather than within the cytoplasmor nucleus.

Rochalimaea quintana is the causative agent of Trench fever, a disease that decimated troops in world war I.

Species genus Ehrlichia cause disease in human and other animals, two of which are ehrlichiosis in human and Potomae fever in horse.

Resistance:

Rickettsias are susceptible to chemical disinfection and are destroyed by heat and dehydration. Antibiotics such as chloroamphenicol and tetracyclin inhibit their growth. Inhibit protein synthesis through interference with the binding of amino acids tRNA to 30S sub 20 unit water for once with the binding as Amino acet t-RNA to 30S sub cm.

Actinomycetes:

Description of actinomycetes is found in volume 4 of second edition of Bergey’s Manual. They are aerobic, gram positive bacteria that form branching filament or asexual spore. Actinomycetes are divided into different groups on the basis of their morphology, arrangement of spores, wall chemistry and type of sugar preset in cell wall.

Actinomycetes have considerable practical significance. They are most restricted in soil inhabitance. They can degrade enormous and a variety of organic compounds. Actinomycetes take part in mineralization of organic matter in the soli and synthesis of antibiotics.

General Characters of Actinomycetes:

1. Actinomycetes grow on solid substrate such as agar, the branching network of hyphae developed by actinomycetes growing both on the surface of the surface of substratum and below the substratum. Hyphae are divided by septa into long cells containing several nucleoids.

2. Many actinomycetes have an aerial mycelium that extends above the substratum form, asexual, thin wall, spore conidia and condidiospore on the end of filament.

3. Spores develop by septum formation at filament usually in nutrient deprivation.

4. They are not heat resistant.

5. Cell wall composition of actinomycetes vary among different groups. Cell wall type can be distinguished according to 3 features of Peptidoglycan composition.

(i) Amino acid in tetrapeptide side chain position 3^rd.

(ii) Presence of glycine in interpoptide bridge

(iii) Peptidoglycan sugar content.

(i) Nocardiform Actinomycetes:

Bergey’s manual of determinative bacteriology divides 19 genera into 5 subgroups of nocardiform. These bacteria develop a substrate mycellium that readily breaks into rods and coccoid elements. Several genera also form an aerial mycellium that rises above the substrate and may produce conidia.

All genera have high G + C content like other actinomycetes and ail are strict aerobes. The cell wall is made up of arabinose and galactose. Mycolic acid is present in genera nocardia and rhodococcus.

(ii) Actinomycetes with Multicellular Sporangia:

It forms cluster of spores when hyphae divide both transversely and longitudinally. All Three genera in this group have type III cell wall, and sugar pattern is different in three genera. GC content varies from 57-75%.

(a) Geodermatophilus (Type III C):

16 form motile spore and is an aerobic soil organism.

(b) Dermatophilus (Type III B):

It form motile spore with tuft of flagella but it is a facultative anaerobe and causes streptotrichosis disease.

(c) Frankia (Type III D):

It forms non-motile sporangiospore. It grows in symbiotic association with the roots of higher non-leguminous plants (Alder tree) and is able to fix nitrogen. Frankia form branching hyphae with globular vesicles at the end.

(III) Actino Planates:

(i) They grow almost in soil habitat, Pilemia grow in association with Keratin.

(ii) Mircomonospora actively degrades chitin and cellulose.

(iii) It can provide antibiotics such as gentamycin.

(iv) They have substrate mycelium and wall type II D.

(v) Aerial mycelium is absent.

(vi) Conidiosphores form at the end of hyphae called sporangiophore.

(vii) Spore can be motile or non-motiie.

(viii) Actinoplane and Pilemelia have spherical, cylindrical and irregular sporangia. Sporangia develop above the substratum at the tip of sporangiophore.

Dactylosporangium:

It forms club shaped, finger like or pyriform sporangia with 1-6 spores.

(iv) Streptomyces and Related Genera:

(1) They have aerial hyphae divided in single plane to form chain 5-15 or more non-motile conidiospores.

(2) All have type I cell wall and G + C content around 69-78%.

(3) They are strict aerobes.

(4) They are useful in use of carbohydrates, antibiotics production, milanin synthesis, nitrate reduction and hydrolysis of urea.

(a) Verticillium:

It has an aerial mycelium with the whole of 3-6 short branches, These branches have secondary branches bearing chain of spores.

(b) Spirochyta:

It is one of the strangest actinomycetes. It lacks substrate mycellium. The hyphae remain attached to the substratum by holdfast and grow upward to form aerial mycelia. They release mobile, flagellate conidia in the presence of water.

Names of some antibiotics which are introduced by streptomyces;

Chloroamphenicol — S. venejualis

Tetracyclin — S. aureofaciens

Kanomycin — S. kanamyceticus

Teramycin — S. rimosus

Neomycin — S. fradiae

Erythromycin — S. erythreus

Antifungal:

Nystatin — S. nourrei

Amylase — S. vulgaris

Protease — S. noursei

(v) Maduromycetes:

1. Actinomadura, Microbiospora, Planospora, and streptosporangia associated with disease actimycetoma.

2. All genera have type III cell wall and sugar derivative madurose.

3. G + C content is 64-74%.

4. Aerial mycelia bear pairs or short chain spore.

5. Substrate mycelia are branched.

Thermomonospore:

(1) It has been isolated from high temperature habitat. They can grow at 40-48°C.

(2) All genera have type III cell wall and type C sugar pattern.

(3) Thermomonospora produce single spore on aerial mycelia or both aerial or substrate mycelia.

Thermoactinomycetes:

(1) Thermoactinomycetes is thermophillic and wall type lllc grows between 45-60°C.

(2) It forms single spore on both aerial and substrate mycellium.

(3) Its G + C content is lower than that of other actinomycetes 52-55%.

(4) Thermoactinomycetes spores are free endospore. They can survive at 90 °C for 30 minutes.

Cyanobacteria:

Cyanobacteria are otherwise called Blue-green algae or blue green bacteria. Terms such as cyanophta, myxophyta, cyanochloronta. etc. are also used to denote Blue green algae.

They represent a group of photosynthetic, mostly photolysis mediated oxygen-evolving monomers (Prokaryotes).

Cyanobacteria represents a “connecting link” between bacteria and green plants.

These are similar to bacteria in:

(a) Being prokaryotic

(b) Absence of well-defined nucleus, chloroplasts and mitochondria

(c) The cell wall chemistry

(d) The mode of cell division

(e) Have slimy covering

(f) The absence of any sexual stages in the life history

(g) Both posses reserve food material

Cyanobacteria are similar to green plants and algae in:

(a) They have Chlorphyll a and photosystem II, and carry out oxygenic photosynthesis.

(b) Their principle mode of mutablism

(c) Their contribution to natural nutritional cycle

Characteristic Features of B.G.A:

(1) These are oxygenic, gram negative prokaryotes without well-defined nucleus.

(2) Cytoplasmic membrane, the genetic material, the photosynthetic apparatus and respiratory system are not segregated by means of internal membranes, from the rest of cell.

(3) They are both microscopic and macroscopic, some are visible to the naked eye because of their mass occurrence.

(4) The main cell wall constituent of cyanobacteria is peptidoglycan.

(5) Outside the cell wall there is a gelatinous shealth.

(6) Vacuoles are formed.

(7) They show great diversity in form and shape; some are rod shaped; few of them are unicellular or multicellular.

(8) Unicellular form are non-motile but most filamentous cyanobacteria show a gliding motility at some stage of development and lack flagella.

(9) Some cyanobacteria have capability to fix atmospheric nitrogen.

(10) Nucleus is like prokaryotic and is devoid of nuclear membrane.

(11) The cyanobacterial cytoplasm is traversed extensively by flattened vesicular structures called thylakoids or lamellae, the photosynthetic site.

(12) The principal photosynthetic pigment of all cyanobacteria is Chlorophyll a. Besides, they possess β-carotenes another accessory pigments, namely phycobiliprotein. The phycobiliproteins are phococyanin, allophycocyanin, β, and phycoerythrin [water soluble pigment].

(13) Stored food is cyanophycin starch.

(14) In some forms thick wall akinites are also formed which help them to serve in unfavourable conditions.

(15) In trichome of cyanobacteria, heterocyst are observed which help in fixation of N₂.

(16) They have no TCA cycle due to absence of key enzyme, α-ketoglutarate dehydrogenase.

(17) Some cyanobacteia are symbiotic.

(18) None of the cyanobacteria is known to cause human infectious diseases though some produce toxic substances.

(19) Some are thermophilic and grow in hot-springs.

Fossil History:

Cyanobacteia are three billion years old and their fossils appear in early Pre-cambrian period. These were the first to release elemental oxygen in to primitive atmosphere. These were responsible for major evolutionary transformation leading to the development of aerobic metabolism and to the subsequent rise of higher plants and animal forms.

Distribution in Nature:

Nostoc live in lichen Peltigera and in the roots of cycas. Anabaena azollae grows in leafy cavity of tropical aquatic fern Azolla. Cyanobacteria includes about 150 genera and 250 species. They form the component of marine and freshwater phytoplankton. They are found as saparophyte and parasite.

They form gllatinous and powdery coverings on the substratum. The colour of these form may vary from blackish green to olive green orange yellow or redish brown, besides the typical bluegreen. Blue green algae are also found in moist soil. They also occur in hot springs (e.g., Synechococcus lividus and live at pH 4 and 70°C).

Forms of Cyanobacteria on the Basis of Shape Morphology and Classification:

The cyanobacteria include unicellular and multicellular forms and can be divided into five groups on the basis of their morphology:

Group-I:

Unicellular rods and cocci are collected in the group chroococcal cyanobacteria. The cells are seen either as individuals or as aggregates which are kept together by capsules or slime. Cell division occurs only by binary fission or budding. This group includes Synechococcus (formerly Anacystis niduloms), Gloeocapsa, Gloeothece and Gloeobacter violaceus.

Group-II Pleurocapsuiar Cyanobacteria:

Also includes unicellular forms, but only those which can also multiply by multiple fission. During this process, many small cells called baeocytes, appear within dividing mother cell. e.g. Pleurocapsa, Dermocarpa, and Myxosarcina etc.

Filamentous Blue Green Algae:

The following three groups are characterized by threadlike cell aggregates, they from trichome (chain of cells). Growth is intercalatary, that is by cell division within trichome. The trichome exhibit gliding motility. Multiplication also occurs through break-up of the trichomes and formation of homogonia. For this reason, these filamentous blue green algae have also been called homogonal blue green algae.

Group-III Filamentous Cyanobacteria without Heterocyst:

Trichomes consist only of vegetative cells. Typical of this group are Oscillatoria, Spirulina, lyngbya, Phormidium and Plenctonema.

Group-IV:

Filamentous cyanobacteria with heterocyst: When trichomes are grown without fixed nitrogen. They differentiate in to heterocysts. In some cases, akinetes (thick walled, resting cells) may occur e.g., Anabaena, Nostoc and Calathrix.

Group-V:

Filamentous cyanobacteria with heterocyst:

The members of this group differ from those of the previous group by their cell division in more than one plane, e.g., Fischerella.

Ultrastructure of Cyanobacterial Cell:

Mucilage Sheath:

Cells of most of cyanobacteria have their (Anacystis) or thick (Anabaena) mucilaginous sheath or capsule called slime layer whose thickness, pigmentation, consistency and nature is greatly influenced by environmental factors. It is considered that these microorganisms secrete the mucelage through pores present in their cell walls.

Cell Wall:

The cell wall lies between the plasma membrane and mucilage sheath. Like bacteria, peptidoglycan is the main constituent of the cyanobacterial cell. Ultra structurally, the wall consists of four layer (LI, LII, LIII, & LIV) each of which is connected with the other one by a connection known as plasmodesmata. Some cyanobacterial cell walls like other monera, possess one substance called diaminopimelic acid.

Plasmamembrane:

Cyanobacterial plasma membrane is 70A° thick. It is selectively permeable, lacks sterole such as cholestrol, and consists of the proportion of protein to phospholipids high (2:1) like other monerans. It generally fuses with photosynthetic lamellae and gives rise to inward folding’s in the cytoplasm, called lamellosomes or mesosomes.

Cytoplasm:

Cytoplasm of cyanobacterial cell, like bacteria lacks cell organelles such as chloroplasts, mitochondria, ER and golgi bodies. But it possesses photosynthetic apparatus, ribosomes and large number of subcellular inclusions such as a granules, p granules, photo phosphate bodies, polyhedral bodies, cyanophycin granules are the genetic material.

Cytoplasm can be divided in two parts:

Chromatoplasm:

This is peripheral part of cell which contains thylacoid or lamellae that are arranged in parallel rings or are scattered. They are sac like structure enclosed by unit membrane. Phycobilisome or biliprotein are not apart from these. Cytoplasm contains ribosomes, cyanophycin, starch granules, carboxysomes, polyhedral bodies, and gas vacuoles.

Centroplasm:

Central transparent part of cell is called Centroplasm. The genetic material is formed in this protein portion. Genetic material is in the form of DNA. Besides DNA, RNA is also present. Thus in cyanobacteria organised nucleus is not present and this nucleus is called incipent nucleus.

Photosynthetic Apparatus:

Photosynthetic apparatus is present in the form of thylakoids which occur either parallel to the cytoplasmic membrane or coiled at the periphery of protoplasmic space. The thylakoid membrane contains chlorophyll a, β-carotene and oxo-carotenoids like myxoxanthophyll, echinenon and zeaxanthin as well as components of photosynthetic electron transport system .

The special feature of the thylakoids of cyanobacteria are (phycobilisomes, disc like structure attached to outer surface of thylakoids.

They consist of phycobiliprotein, phycocyanin (75%), allophycocyanin (12%) and phycoerythrin, and some colourless polypeptides. The last two component constitute about 12%. The phycobiliprotein in tern are composed of proteins and their prosthetic groups, phycocyanotrilin and phycoerythrobilin.

The accesary pigments transfer light energy they absorb to chlorphylla. They also have a protective shading junction that prevents oxidation in intense light of other photosynthetic pigment.

Note: Only one of the cyanobacteria, Gloeobacter violaceus lacks thylacoids and phycobilismes. It has chlorophyll localised in cytoplasmic membrane and phycobiliprotein are attached as a continuous layer to the internal surface of the cytoplasmic membrane.

Cellular Inclusions:

All cyanobacteria are apparently capable of accumulating polysaccharides in the form of glycogen granules and phosphate as polyphosphate. These are the spherical structures formed as a result of the aggregation of high molecular weight linear polyphosphate.

These sub-cellular inclusions are also called as mitochomation granules or volutin granules and serve as phosphate stores and are consumed during periods of phosphate starvation. These structure develop mostly in those cyanobacteria which grow in phosphate-rich environment.

Cyanophycin Granules:

These are storage materials that are found in cyanobacteria. Cyanophycin are large bodies composed of store protein in the form of polypeptide and contain aspartic acid and arginine in a ratio of 1: 1. This polymer apparently junction as a nitrogen reserve.

It decreases under nitrogen starvation and increases again on addition of a nitrogen source. It is stored predominantly in heterocyst. However, it may also function, as energy store, since arginine can serve to regenerate ATP under anaerobic condition, via breakdown in to ornithine and carbamoyl phosphate.

Polyhedral Bodies/Carboxysomes:

All cyanobacteria store their ribulose 1, 5-diphosphate carboxylase (RUDP carboxylase) enzyme in structure referred to as polyhedral bodies.

Gas Vacuoles:

It is made up of gas vesiles. They do not contain true protein-lipid membrane. The membrane is permeable to gases and able to exclude water, and maintain buoyancy.

Nutrition:

Cyanobacteria like other algae are able to prepare their food through photosynthesis but they are different from phososynthetic bacteria which do not produce oxygen during photosynthesis.

Reproduction:

Cyanobacteria reproduce by vegetative or asexual reproduction. True sexual reproduction is not present.

Vegetative Reproduction:

It occurs by following methods:

By Binary Fission:

Unicellular cyanobacterial cells divide and reproduce by binary fission.

By Fragmentation:

In this method fragments breakdown in to small pieces and each piece develops in to a new colony.

Asexual Reproduction:

1. Hormogonia:

Filament break at number of places into smaller pieces, called as hormogonia by the death and decay of ordinary cell. They slip out of mucilaginous sheath and grow into new plant. Frequently, the trichomes break near heterocyst.

Non motile cyanobacteria reproduce by spore which are of the following types:

By Baeocytes:

They are small, spherical reproductive cells of the pleurocapsulated cyanobacteria. They are formed by multiple fission within a considerably enlarged cell that is surrounded by a thick fibrous exopolysaccharide layer.

By Endospore:

In this condition one or more cell increases in size and their protoplast divides into many parts and forms endospore. Endospore forming cell behaves as a sporangium. Endospores are generally naked, but a thin wall is secreted after their liberation from sporangium.

By Exospore:

In some epiphytic form the delicate cell-wall ruptures apically exposing the protoplast from which spherical spores are abstricted successively from its tip, these are called exospores. The abstricted spores are surrounded by a delicate membrane. The exospores may germinate when already attached to the parent protoplast giving rise to new individual.

Akinetes: [Resting Spore]:

These are yellow or brown coloured structure formed from vegetative cell, under certain condition enlarges and accumulates food material and develops thick walls. These are called akinetes and may be arranged on either side of the heterocyst or in between two heterocyst.

In mature akinetes, the outer well may be 2 to 3 layered and its protoplasm becomes highly granular. The akinetes geminate under favorable condition and their content liberated out through a pore and protoplast by further division form filament.

Heterocyst:

All filamentous form of cyanobacteria possess heterocyst. Each heterocyst has two walls, outer wall is thick and inner wall is just like a membrane. They may be intercalary and basal in position. In case of intercalary two pore or two polar nodules are there.

Heterocyst are larger than vegetative cells and appear empty in the light microscope, while akinete appear full of storage product.

The heterocysts are produced from vegetative cells:

1. After dissolution of storage granules.

2. Deposition of multilayered envelope outside the cell wail.

3. Damage and breakdown of photosynthetic thylacoids.

4. Formation of new membrane structures. The formation of heterocyst is inversely related to amount of N₂ in the algae.

5. They are large, thick walled, empty cell.

Heterocyst envelope contains:

Carbohydrate = 62% [cellulose absent]

Lipid = 15%

Amino compounds = 4%

Polyphosphate = Absent

Pigment = Chlorophyll ‘a’, caroteroids

They contain nitrogenase enzyme which fixes atmospheric nitrogen.

Heterocyst lack the photosystem needed to generate oxygen, which helps to maintain their cytoplasm anaerobic. (Oxygen inhibits nitrogen are activity).

The MoIIicutes (Mycoplasma):

The common name for this group has traditionally been the mycoplasmas. However, this usage inertes confusion, since it is often not clear whether “mycoplasma” is being used generically or refers to a number of the genus Mycoplasma. We will, thus, adopt the more recently coined term mollicutes as the designation for member of the larger groups.

The distinguishing property of the mollicutes group of eubacteria is their lack of cell wail.

Consequently, they share a unique constellation of characteristics:

Sensitivity to osmotic lysis, resistance to penicillin and other antibiotics that inhibit cell wall synthesis, pleomorphic shade and easily deformable cells that allow them to be squeezed through membrane filters with a pore size small enough to retain most walled bacteria.

All mollicutes show the additional properties of being parasites of eukaryotic organisms, and having complex growth factor requirement that typically includes fatty acid, amino acids, purines and pyrimidines, vitamins and sterols in all but A. choleplama spp. The fatty acids and sterols are usually provided by supplementing media with serum, which contains these compounds as soluble, non-toxic lipoprotein.

Colonies that develop on solid media are often small, in some cases, microscopic. They typically have “fried egg” appearance. The raised center is a nearly spherical mass of cell pontly embedded in agar. It is surrounded by a thin film of surface growth.

Although the mollicutes lack a cell wall they all have substantial amount of polysaccharides associated with cell membrane. The detailed structure of this material is not known for any member of the group. However, it appears, in all cases, to be composed principally of hexoses, after including hexosamine particularly glucosamine and galactosamine and N acetyl glucosamine.

This layer has been termed as capsule, but it is difficult to remove from the membrane, suggesting that it may be covalently bound to hydrophobic constituents. Probably it plays and a structural role, partially compensating for the lack of peptidoglycan wall. A similar role has been proposed for the lipopolysaccharides of the wall-less archaebacterium, Thermoplasma.

The evolutionary origin of this group was obscure until recently. It was presumed to be polyphyletic, i.e., the group was presumed to contain organisms derived as stable L—Form from a variety of different bacteria. However, many similarities among mollicutes, including usually low G+C value (23 to 36%) and small genome size (mostly 0.5 x 10⁹ to 10 x 10⁹) suggested that they have common orgin.

This suspicion has been confirmed by 16S r RNA sequencing: The mollicutes are a coherent phylogenetic group closely related to the Clostridia.

Five genera are recognized in mollicutes:

Metabolism of Mollicutes:

With a single exception (ureaplasm) the mollicutes are fermentative. Growth substrates include carbohydrates and amino acids, particularly arginine, the metabolism of which proceeds via the arginine dihydrolases or arginine deiminase pathway, a pathway that also occurs in a variety of other bacteria.

Mechanism of “arginine dihydrolase” reaction which permits the generation of ATP by substrate level phosphorylation.

The failure of mollicutes to respire reflects their lack of cytochromes and quinine. However, many mollicutes do have a rudimentary electrons transport chain that consists of a flavoprotein coupled NADH oxidase that reduces O₂ directly. Only in acholeplasma, this is a membrane bound system. In other members of the group, it is cytoplasmic.

There is no evidence that a proton motive force is generated by electron transport even by Acholeplasma presumably, the function of the oxidase is to re-oxidise the NADH formed during carbohydrate fermentation.

Ureaplasma:

In Ureaplasma, energy is conserved via a chemiosmotic mechanism. This organism depends on the hydrolysis of urea for its growth. The mechanism of proton pumping is not fully established. Hydrillaysis of urea generates ammonia which leaves the cell as ammonium, thereby carrying a proton with it.

Cell Shape and Reproduction:

All mollicutes are pleomorphic to at least, some extent. However, it is becoming clear that much of the extensive pleomorphism observed by early workers was a result of sub – optimal growth condition. Several members of this group exhibit a fairly constant morphology and reproduce by binary fission.

Indeed one of them (Spiroplasma) grown as quite regular spirals. Others (Ureaplasma, Anoeroplasma) are typically unicellular cocci or coccobacilli (very short rods ). Only in mycoplasma and Acholeplama do healthy growing cells exibit extensive variation in form.

In these organisms reproduction takes two forms: unicellular cocci may divide by fission or they elongate into branching filaments that then, fragment into many cocci. Culture of mycoplasmas and Acholeplama thus may contain a mixture of cocci, short filaments and longer branched filaments.

Human diseases: Primary Typical Pneumonia (PAP) characterized by fever, cough, headache and Pneumonia Non gonococcal urethrites (M. hominis). Infertility of women, Arthistis or otitis media, Inflammation of middle ear.

Mycoplasma:

The mycoplasmas are parasites of animal mucous membrane principally those of respiratory or genital tract and the synovial membrane of Joint capsules.

Infections may cause disease, e.g., Pneumonia or Arthritis.

They are facultative anaerobes and some prefer redused O₂ tensions.

Most ferment either arginine or carbohydrates; some are capable of fermenting both.

The product of carbohydrate fermentation always includes lactate with variable but usually small amount of acetate, pyruvate and butanediol?

The genus is quite larger; over 60 species are currently recognized.

A few mycoplasmas are capable of unidirectional gliding movement.

They have polarized cell organization that includes, at the anterior end of cell, protrusion of the cell membrane surrounding a rod like structure. The rod consists of a bundle of parallel proteinaceous fibers. There is some evidence suggesting that constituent proteins are actin like.

Acholeplasma:

The Acholeplasma are also animal parasites, and may be widely distributed in the tissues of may different vertebrates.

They have consequently been a nuisance in the in-vitro culture of animal cells: because the prevention of bacterial contamination of tissue culture has usually relied on the inclusion of penicillin in the medium; a measure ineffective against mollicutes. Such contamination can damage the cultured animal cells; because, the production of H₂O₂ by NADH oxidase of contaminating acholeplasma may be cytotoxic.

Acholeplasma is sharply distinguished from all other mollicutes by its ability to grow in the absence of sterols. However, if sterols are available, Acholeplasma will incorporate them into the cell membrane.

The independence from exogenous sterols does not reflect on ability to synthesize them; rather, it reflects the ability of these mollicutes to organize their members as functioning, stable structure, without the strengthening effec  of sterols, some acholeplasmas synthesize and incorporate into their membranes large amount of carotenoids which serve a sterol like function; others apparently do not have a sterol replacement in their neutral lipid fraction.

Spiroplasma:

The genus Spiroplasma contains mollicutes that are parasitic on arthropods and plants. They are particularly common in the haemolymph, gut and salivary glands of insects like leafhoppers, that feed on plants. Presumably plants become infected during feeding on these insects. A wide variety of plant diseases are caused by spiroplasma infection of plants vascular tissue.

These organisms are also encountered in arthropods, e.g., rabbit ticks, that feed on animals and in some cases may disease in experimentally infected laboratory animals consequently there is a growing suspicion that spiroplasma may be important aetiological agents of human and animal diseases.

Spiroplasma is a helical, normally less than 0.2 um is diameter, and 3 to 4 lam long. They are motile with same range of movements that are characteristic of spirochete: swimming, flexing and creeping. In wet mounts, they are easily mistaken for spirochetes: only by observing them in electronic microscope, they can be distinguished by their lack of cell walls and axial filament.

Anaeroplasma:

The anaeroplasma are strictly anapestric mollicutes that inhabit the bovine and ovine rumen. They ferment carbohydrates to a mixture of acids (acetate, format succinate, lactate and’ propionate) ethanol and CO². Some are bacterialytic; they lyse walled bacteria by excreting lytic enzymes.

Their lipid requirement are complex and poorly understood. Sterols are required, as esterified fatty acids in the form of phospholipids or lipopolysacchaes plasma smaller independently replicating prokaryote.

(1) Autoreplication

(2) Smaller genome

Ureaplasma:

The Ureaplasmas are only non-fermentative mollicutes. They are microaerophiles and depend on the hydrolysis of urea for their energy. They never grow to high cell densities; 10⁷ cells per milliliter in characteristic of growth in liquid media. Ureaplasmas normally inhabit the mouth and respiratory genital tracts of human and animals.

Distribution and Species:

The members of mollicutes are parasitic bacteria. They do not kill their hosts, but they produce predominantly chronic infections, and in this sense they are very successful parasites.

In animal they may appear as harmless parasites on the serous epithelia of respiratory and genital tracts (in mammals and birds). They are membrane parasites in that they adhere tightly to the epithelial cells of the serous membranes.

They do not excrete toxin but because of the intimate contact between parasites which does not have the cell wall and the host cell, even weakly toxic metabolite like ammonium ions and hydrogen peroxide may have toxic effects on the host cells.

In plants Mycoplasmas cause yellowing disease. They are predominantly localised in phloem, part of the vascular system. Spiroplasma citri causes a yellowing disease in citrus tree. Spiroplasma has also been found in bees and grasshoppers and it may be assumed that insects are not only carriers but also hosts of Spiroplasma species.

Biochemical Properties:

Mollicutes are differentiated from all other bacteria not only by the lack of a cell wall, but also biochemical characters. The organisms grow only in isotonic or hypertopic media (with sorbitol or sucrose ) and require purines and pyrimidine as well as lipids and steroids. The lack of quinine and cytochrome suggests a very limited respiratory chain.

Klienberger Relation to L-Form:

A strain of Streptobacillus moniliformis that grew as irregular protoplast was isolated in 1934. These cells were called L from the Lister Institute in London where they were isolated. The loss may be complete or partial (some have defective wall) and parent organism may be either gram positive or gram negative. They are pleomorphic like mycoplasmas and continue to reproduce.

These organisms can arise through spontaneous mutation or from treatments such as growth in isotonic or hypertonic media containing penicillin. If all traces of peptidoglycan disappear, bacteria cant not resynthesize it because pre existing wall is necessary to construct new peptidoglycan.

In this case two types of L-forms have been isolated: Labile form, which reverts back to normal cells with complete cell walls on cultivation without penicillin and stable form which does not form cell walls even in the absence of penicillin. It was first assumed that the mycoplasma types arose by mutation of normal bacteria to stable L-forms.

However, genome size and GC content of Mycoplasma contradict their origin from eubacteria and suggest that they constitute a separate class.

L-forms are not closely related to Mycoplasma and should not be confused with them.

Structure of Mollicutes:

i. Nudear structures are less evident than normal bacteria.

ii. Mesosomes are absent.